1. Field of the Invention
The present invention relates to a method and an apparatus for manufacturing a photomask, as well as to a method of manufacturing a semiconductor device. More specifically, the present invention relates to a method and an apparatus for forming a photomask which is used as a master of a transfer pattern in a process of manufacturing a semiconductor integrated circuit, and to the method of manufacturing the semiconductor device.
2. Description of the Prior Art
Elements forming semiconductor integrated circuits have recently been miniaturized to a higher extent. Miniaturization of the semiconductor integrated circuit is allowed owing to photolithography by which a fine pattern can be formed with high accuracy. For miniaturizing the pattern, a wavelength of exposure light is reduced, and a numerical aperture of a reducing projection exposing device (which will be referred to as a "stepper" hereinafter) is increased. According to the current technology, a device using KrF laser (wavelength of 248 nm) and having a pattern of 0.25 .mu.m in width is available.
Although miniaturization of the pattern has been developed as described above, it is essential for increasing a degree of integration to overlay or stack a plurality of patterns with high accuracy. This overlay accuracy may be lowered due to an expansion strain (specifically expansion and contraction strain), a rotational strain and a field strain of a stepper. According to the current technology level, the expansion strain and rotation strain can be removed by a correcting function of the stepper itself. Therefore, a cause which may actually lower the overlay accuracy is principally the field strain of the stepper. This field strain maintains a substantially constant value when exposure is carried out with the same stepper and under the same optical conditions (numerical aperture and lighting conditions). Therefore, a difference in field strain between first and second patterns is sufficiently small, when the first pattern is formed on a semiconductor substrate with a first stepper under first optical conditions and the second pattern is formed with the same stepper under the same optical conditions, so that any problem impeding high integration does not arise.
However, a difference in field strain causes a problem when different steppers are used and/or different optical conditions are used.
FIG. 18 is a schematic view of a first stepper for showing a field strain. Referring to FIG. 18, a first stepper 1102 includes a light source 1103, a photomask 1105, a reducing lens 1106 and a stage 1108. Light source 1103 emits KrF laser beams represented by arrows 1104. Photomask 1105 is provided with a predetermined mask pattern 1105a. Reducing lens 106 converges the KrF laser beams. A wafer 1107 is laid on stage 1108.
In this first stepper 1102 thus constructed, the KrF laser beams emitted from light source 1103 and represented by arrows 1104 pass through a portion of photomask 1105 not provided with mask pattern 1105a. The KrF laser beams passed through photomask 1105 are converged by reducing lens 1106, and thereafter reach wafer 1107.
FIG. 19 is a plan taken along line XIX--XIX in FIG. 18. Referring to FIG. 19, the KrF laser beams represented by arrows 1104 are radiated to wafer 1107 on stage 1108. Deviations or shifts of magnitudes and directions indicated by arrows 1109 occur between ideal pattern positions, which are determined by an ideal mask and an ideal optical system, and actual pattern positions, which are determined by an ideal mask but by an actual optical system. It is assumed that these deviations do not contain deviations caused by parallel movement, rotation, extraction and contraction. The deviations represented by arrows 1109 are called field strains.
FIG. 20 is a schematic view of a second stepper for showing the field strains. Referring to FIG. 20, a second stepper 1202 includes a light source 1203, photomask 1105, a reducing lens 1206 and a stage 1208. Light source 1203 emits KrF laser beams represented by arrows 1104. Reducing lens 1206 converges the KrF laser beams represented by arrows 1104. Wafer 1107 is disposed on stage 1208. Second stepper 1202, light source 1203, reducing lens 1206 and stage 1208 are different from first stepper 1102, light source 1103, reducing lens 1106 and stage 1108 shown in FIG. 18, respectively. The KrF laser beams represented by arrows 1104 in FIG. 20, photomask 1105, mask pattern 1105a and wafer 1107 are the same as the KrF laser beams represented by arrows 1104 in FIG. 18, photomask 1105, mask pattern 1105a and wafer 1107, respectively. The KrF laser beams represented by arrows 1104 and emitted from light source 1203 pass through a portion of photomask 1105 not provided with mask pattern 1105a. The KrF laser beams passed through photomask 1105 are converged by reducing lens 1206, and thereafter reach wafer 1107 on stage 1208.
FIG. 21 is a plan taken along line XXI--XXI in FIG. 20. Referring to FIG. 21, wafer 1107 is laid on stage 1108. The KrF laser beams represented by arrows 1104 are radiated to wafer 1107. Similarly to the foregoing case, field strains of magnitudes and directions indicated by arrows 1110 occur between ideal pattern positions and actual pattern positions. Therefore, a positional relationship between the patterns formed by first and second steppers 1101 and 1202 contains an error with respect to an ideal positional relationship. This error is a difference between the field strain caused by first stepper 1102 and the field strain caused by second stepper 1202, and is represented by arrows 1111. The difference between these field strains may have a magnitude of up to about 0.05 .mu.m according to the current accuracy of steppers.
FIG. 22A is a plan showing a conventional interconnection pattern. FIG. 22B shows, on an enlarged scale, a portion surrounded by line B in FIG. 22A. FIG. 22C is a cross section taken along line XXIIC--XXIIC in FIG. 22B. Referring to FIGS. 22A-22C, an interlayer film 2001 is formed on a wafer 2000. An interconnection layer 2002 is buried in interlayer film 2001. A hole 2003 reaching wafer 2000 is formed at interlayer film 2001. Hole 2003 is not in contact with interconnection layer 2002. If interconnection layer 2002 and hole 2003 were formed at ideal positions with ideal sizes, a distance W in FIG. 22C between hole 2003 and interconnection layer 2002 would be 0.1 .mu.m.
However, an error in size occurs in processes of etching interconnection layer 2002 and forming hole 2003. Therefore, distance W cannot be smaller than about 0.06 .mu.m even if they are formed at ideal positions. The foregoing field strain difference represented by arrow 1111 is about 0.05 .mu.m. Therefore, if the pattern positions move due to this difference in the structure where the distance between the patterns is already reduced due to a work error, the distance W may be reduced to about 0.01 .mu.m.
In the actual process, deviation of up to about 0.05 .mu.m may occur due to parallel movement and/or extraction (or contraction) caused by errors in detection of a base mark and movement of stage. If this deviation is added to the foregoing error, hole 2003 is brought into contact with interconnection layer 2003. Therefore, it may now be difficult to provide overlaid fine patterns with different steppers.
In order to overcome the above problems, the same stepper may be used to provide overlaid layers, i.e., a first layer to be formed according to a first resist pattern and a second layer to be formed on the first layer according to a second resist pattern. In this case where all the layers are formed by the same stepper, such problems arise that the required steppers increase in number and a processing time increases.
Even if the same stepper is used, the first and second layers may be formed under different optical conditions. In this case, a relative difference occurs between the field strain of the stepper in the process of forming the first layer and the field strain of the stepper in the process of forming the second layer. For example, the interconnection layer may be formed by radiation of high .sigma. rays, and the hole layer may be formed by radiation of low .sigma. rays. In this case, it has been found that overlay deviation of up to about 0.03 .mu.m occurs due to a relative difference between the field strains. Therefore, it is impossible to improve the overlay accuracy.